1. Field of the Invention
This invention relates to a method of displaying a radiation image for medical purposes or the like on a display device such as a cathode ray tube (CRT) by conducting an unsharp masking process on the radiation image, and an apparatus for carrying out the method. This invention also relates to a method of calculating an unsharp mask signal for use in the unsharp masking process, and an apparatus for carrying out the method.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays of ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation passing through an object to have a radiation image of the object stored thereon, and is then scanned with stimulating rays which cause it to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electrical image signal, which is processed to reproduce a visible image having an improved image quality, particularly high diagnostic efficiency and accuracy.
The aforesaid radiation image recording and reproducing system is very advantageous in that it is possible to obtain a visible radiation image suitable for viewing, particularly for diagnostic purposes, by photoelectrically reading out a radiation image stored on the stimulable phosphor sheet, conducting appropriate signal processing on the electrical image signal thus detected, and reproducing the visible radiation image on a recording material such as a photographic film or on a display device such as a CRT by use of the processed electrical signal.
As one of the methods of processing the electrical signal, an unsharp masking process method has been proposed as disclosed, for example, in U.S. Pat. No. 4,315,318.
In brief, the proposed method is characterized by obtaining an original image signal (electrical signal) Sorg by scanning a radiation image stored on a stimulable phosphor sheet with stimulating rays and photoelectrically reading out the radiation image, obtaining an unsharp mask signal Sus corresponding to a super-low frequency at each scanning point on the stimulable phosphor sheet, performing a signal conversion represented by the formula EQU S'=Sorg+.beta.(Sorg-Sus)
where .beta. denotes an emphasis coefficient, to emphasize the frequency components (frequency region) above the super-low frequency, and using the signal S' for the formation of a visible image.
The unsharp masking process method is very advantageous for enhancing the contrast of the image of an object such as a lung blood vessel, a bone end portion, a tumor or soft tissue, for improving diagnostic efficiency and accuracy, and for decreasing the disease oversight ratio.
However, the conventional method of displaying the visible image by conducting the unsharp masking process has drawbacks as described below.
Namely, when a visible image is displayed by conducting the unsharp masking process, the image has heretofore usually been output and displayed as a single still image on a film, a hard copy or a CRT. Therefore, on a single output image, the result obtained by emphasizing only one frequency region by only one emphasis coefficient is displayed.
However, appropriate values of the emphasized frequency region and the emphasis coefficient differ in accordance with the viewing purposes, particularly the diagnostic purposes, even in a single image. For example, in the case of a limb bone fracture, it is desired to emphasize a comparatively high frequency region and express the bone contour sharply to accurately diagnose the fracture line or the like. On the other hand, in order to diagnose inflammation of the soft tissue around the bone fracture, it is desired to emphasize a comparatively low frequency region.
Accordingly, in the case where diagnosis should be conducted by performing a plurality of unsharp masking processes under different conditions (different emphasized frequency regions and/or different emphasis coefficients) for a single image, there have heretofore been employed (1) a method of manually adjusting the unsharp masking process conditions, reproducing differently processed images one by one on films, hard copies or a CRT, and finding the optimal processing conditions, or (2) a method of displaying a plurality of differently processed images side by side on one film, one hard copy, or one or more CRTs, and using them for diagnosis.
However, with the conventional methods, a long time is required since the process is repeated by changing the processing conditions each time the process is conducted, and the cost becomes high since many films are consumed [particularly, in the case of method (1)]. Also, since the number of images reproduced at one time cannot be limitlessly increased, it is not always possible to display the necessary number of images at one time with the necessary variations of the unsharp masking process conditions [particularly, in the case of method (2)].
Thus in the case where a plurality of images subjected to the unsharp masking processes under different processing conditions are displayed, a need exists for efficiently displaying more processed images, thereby improving the diagnostic efficiency and accuracy and facilitating diagnosis.